This change improves compression ratio for ETC1 encoding.
Explanation:
When computing endpoint weights for ETC1 encoding, it is possible to use delta luma instead of the Euclidean distance between the outer endpoint colors, as it gives approximately the same result.
When computing selector weight, it is important to take into account the following factors:
- The bigger is the difference between the outer endpoint colors, the bigger error can be introduced by the corresponding selector, therefore the bigger should be the weight of that selector. In the original Crunch algorithm, the selector weight is proportional to the squared distance between the outer endpoint colors. Such optimization improves PSNR, but it might also introduce significant distortion in smooth areas of the image. In order to mitigate this effect, it is proposed to limit the maximum difference between the endpoint colors (currently delta luma is limited by 100).
- Blocks with low difference between the outer endpoint colors introduce relatively small error, so their selectors should have smaller weights. In the original algorithm it is achieved by using squared distance between the outer endpoint colors, though the effect can be amplified further by using powers higher than 2 (currently it is set to 2.7), which improves PSNR.
In the original Crunch algorithm the encoding weights are initialized non-symmetrically (and are set to math::lerp(1.15f, 1.0f, 1.0f / 7.0f) for horizontal split and to math::lerp(1.15f, 1.0f, 2.0f / 7.0f) for vertical split). It is proposed to use the same encoding weight for both splits in case of ETC1 (the used coefficient 0.972 has been computed as math::lerp(1.15f, 1.0f, 1.5f / 7.0f) / 1.15f).
The ETC1 quantization parameters have been adjusted accordingly to preserve the average Luma PSNR.
DXT Testing:
The modified algorithm has been tested on the Kodak test set using 64-bit build with default settings (running on Windows 10, i7-4790, 3.6GHz). All the decompressed test images are identical to the images being compressed and decompressed using original version of Crunch (revision ea9b8d8).
[Compressing Kodak set without mipmaps using DXT1 encoding]
Original: 1582222 bytes / 28.843 sec
Modified: 1473711 bytes / 13.312 sec
Improvement: 6.86% (compression ratio) / 53.85% (compression time)
[Compressing Kodak set with mipmaps using DXT1 encoding]
Original: 2065243 bytes / 36.962 sec
Modified: 1920600 bytes / 18.122 sec
Improvement: 7.00% (compression ratio) / 50.97% (compression time)
ETC Testing:
The modified algorithm has been tested on the Kodak test set using 64-bit build with default settings (running on Windows 10, i7-4790, 3.6GHz). The ETC1 quantization parameters have been selected in such a way, so that ETC1 compression gives approximately the same average Luma PSNR as the corresponding DXT1 compression (which is equal to 34.044 dB for the Kodak test set compressed without mipmaps using DXT1 encoding and default quality settings).
[Compressing Kodak set without mipmaps using ETC1 encoding]
Total size: 1612083 bytes
Total time: 17.351 sec
Average bitrate: 1.367 bpp
Average Luma PSNR: 34.050 dB
Alexander Suvorov
e972e0b480